The present invention relates to techniques of treating items and materials to low temperatures and more particularly, to such techniques that use cryogenic liquids, like liquid nitrogen, to chill items and materials to improve the abrasive wear resistance, corrosive wear resistance, erosive wear resistance and related physical characteristics of the items and materials including metals, metallic alloys, cemented carbides, plastics, ceramics, semiconductors and the like.
Low temperature treatment (-120.degree. F. to -320.degree. F.), or cryogenic processing of materials, particularly metals in the form of cutting tools, has been known to show some improvement in abrasion and corrosion resistance along with reduction of internal residual stresses and improved material stability. Thus, low temperature treatment of metal tools results in improvement in the wear resistance of such tools (increases tool life) whereas the heat treatment of metal tools is utilized to obtain desired combinations of metal hardness, toughness and ductility. With cryogenic processing there is minimal change in the dimension, size or volume of the items treated.
Conventional steel metallurgy is based on the transformation of steel from the relatively soft austenite crystalline state to the harder martensite crystalline state. By heating the steel, it is put into the austenite state and the subsequently rapidly cooling or quenching of the austenite to room temperature triggers a transformation to martensite. Long ago it was observed that more austenite is transformed to martensite if the steel is chilled to below room temperature and when chilled to very low temperature (-120.degree. F. to -320.degree. F.) using cryogenic techniques, the steel hardness and abrasive resistance are greatly improved.
One observer has suggested that merely reducing the few percent of austenite that is left after conventional quenching, by further low temperature chilling to about -300.degree. F., cannot account for the improved hardness and abrasive resistance. That observer claimed that the low temperature chilling produces fine carbide particles that are distributed throughout the martensite and reduce internal stress in the martensite. This explanation may apply to steel and it may apply to some non-ferrous metals, however, it does not apply non-metallic and amorphous materials. For example, copper electrodes are improved by deep chilling to -300.degree. F. and so are nylon violin strings and many other non-ferrous materials. Cryogenic processing has been used for improving the wear resistance of industrial cutting tools, dies, drills, end mills, gear cutters and hand tools such as knives, chisels, planes, saws, punches, files, etc. It has been used to improve durability of turbine blades, ball and roller bearings, piston rings and bushings, and improve the resilience of springs. It has been used to improve performance of resistance welding electrodes and the dimensional stability of castings and forgings. The materials treated have included: steel and steel alloys; titanium and titanium alloys; high-nickel alloys; copper and brass; aluminum and aluminum alloys; cemented carbides; ceramic materials; and a wide variety of plastic materials including nylons and teflons.
Ultralow temperature treatment has been carried out principally using liquid nitrogen as the cooling medium. Temperature descent from ambient temperature to cryogenic temperatures of -300.degree. F. to -320.degree. F. often takes many hours and sometimes several days. The parts, items or materials under treatment are maintained at the low temperature for many hours and then return to ambient temperature over an even greater period and the treatment results are frequently unpredictable and sometimes destructive.
Heretofore, apparatus for chilling small items like tools, electrodes, musical instrument strings, etc., has included a fully insulated box with a removable or hinged top and a payload platform (uniformly perforated) located a short distance above the inside bottom surface of the chamber. cryogenic liquid delivery pipe enters the treatment chamber a point near the top of one of the chamber's side walls and extends downwardly to a point near the bottom of the chamber. The delivery pipe has a liquid discharge port (or extends as a delivery manifold) below the parts platform and introduces the cryogenic liquid to the chamber. The processing cycles may include a sequence of modes of operation including: (a) descent of the payload items into the gas above the cryogenic liquid; (b) further descent into the gas closer to the surface of the liquid; (c) pre-soak for several hours with submersion of parts in the cryogenic liquid of up to 50% to 75% of the maximum cryogenic liquid level height; (d) soak for several more hours with submersion of parts in the cryogenic medium of up to 75% to 100% of the maximum cryogenic liquid level height; and (e) descend fully into the cryogenic liquid which is allowed to evaporate (boil off) until the chamber is free of such medium and the chamber temperature has reached ambient.
Some of the problems encountered with the prior apparatus described above arise as follows: (1) delivery of liquid nitrogen to the bottom of the chamber below the payload platform often splashes or splatters the liquid on the payload parts causing extreme thermal shock to the parts that are still relatively warm; (2) the coldest gas in the chamber is just above the liquid and the gas does not flow upward (rise) to the payload parts--the cold gas does not reach the parts until just about all of the gas in the chamber is cold and the coldest gas will always be below the payload parts; (3) pre-soaking the part partially submersed in the liquid nitrogen causes the part to chill unevenly, as the portion of the part that is submersed chills much faster than the portion that is not submersed; and (4) any submersion of the part in the liquid nitrogen results in boiling heat transfer from the part at an excessive rate that does not allow all portions of the part to cool evenly.